Southern Airboat

0470 blow-by / leak down

0470 blow-by / leak down

Posted: Sun Mar 12, 2017 4:25 pm

by flcracker9

I apologize up front for being long winded. My boat runs great, but getting tired of running around looking like a choo choo train. I went to perform a leak down test today, and after warming up the motor, my piece of chit leak down gauge wouldn't cooperate. My question is 2 part. 1st part is how I ended up performing the test. I removed the pressure gauge and plugged the hole, then I replaced the leak down gauge with the pressure gauge. I opened the manifold adjustment till it read exactly 100psi. My thinking is if I flood the cylinder with 100psi, any leak down would show on the gauge in psi, which in turn would also show as a % since my base was 100psi? I then proceeded to test each cylinder after finding tdc on the compression stroke for each cylinder. The 2nd part of my question regards my readings, and if my testing theory is correct, would like to know your thoughts on the readings I recorded. The 1, 3 & 5 cylinders were supposedly replaced before I bought the motor, and the readings were as follows: #1 = 94psi (6%), #3 = 93psi (7%) and #5 = 95psi (5%), which I believe would confirm these 3 are in pretty good shape. The other side was #2 = 88psi (12%), #4 = 90psi (10%) and #6 = 84psi (16%). Even though the even side was lower, it doesn't appear to me that the readings were all that bad, but it sure does smoke out of the crankcase vent tube. Let me know your thoughts on both my test procedure, and the readings I recorded?Thanks in advance.

Re: 0470 blow-by / leak down

Posted: Sun Mar 12, 2017 5:10 pm

by glades cat

Besides checking the rings for blow-by, you can also check the intake and exhaust for leaks past the valves. Listen to the exhaust pipe and the carb throat for leaks for each cyl. Valve guides can also contribute to oil usage.

Re: 0470 blow-by / leak down

Posted: Sun Mar 12, 2017 8:01 pm

by flcracker9

glades cat wrote:Besides checking the rings for blow-by, you can also check the intake and exhaust for leaks past the valves. Listen to the exhaust pipe and the carb throat for leaks for each cyl. Valve guides can also contribute to oil usage.

Thanks glades cat. I did check the source of the leakage, and it appears the only air escaping on every cylinder was through the crankcase vent. I may be wrong, but if it was a valve, or a guide, it wouldn't cause smoke/pressure from the crankcase vent would it? It also has no smoke from the exhaust at all, and no oil fouling on the plugs. Any thoughts on the readings? Is 16% (the worst cylinder) acceptable (11% more leakage than the best cylinder)? Like I stated, it runs pretty darn good, 72" NGQ set on 2-1/2, spins 3k on the water with no problem, just blows steady smoke out of the crankcase vent tube?

Re: 0470 blow-by / leak down

Posted: Sun Mar 12, 2017 8:24 pm

by John Fenner

Leakdown and compression tests give you a note on which cylinder is out of balance with the others that are stable, by reading your diagnostic numbers it isn't too far off to be worried about it, oil consumption or high oil temperature would be of concern to make an overhaul decision, connies huff alot more than others.

Re: 0470 blow-by / leak down

Posted: Mon Mar 13, 2017 8:23 am

by flcracker9

John Fenner wrote:Leakdown and compression tests give you a note on which cylinder is out of balance with the others that are stable, by reading your diagnostic numbers it isn't too far off to be worried about it, oil consumption or high oil temperature would be of concern to make an overhaul decision, connies huff alot more than others.

Thanks for the reply John. I use about a quart of oil every 15 - 20 gallons, my only concern was the smoke from the crankcase vent tube. Based on the numbers I posted, I should just run it and not worry about the blowby from the vent? I've been running it for almost 2 years like that, just got a little tired of the "hey, you got a lot of smoke coming out of your vent tube" comments.

Re: 0470 blow-by / leak down

Posted: Mon Mar 13, 2017 1:08 pm

by John Fenner

You can slow down consumption and blowby a bit by running some Lucas oil stabilizer in it.

Re: 0470 blow-by / leak down

Posted: Tue Mar 14, 2017 8:04 am

by mojoe

If you are gonna leave it till it lets you know when you gott fix it and still want to shut up the masses, get a vinyl decal made that says something witty and put it right where the smoke comes out: